I do not think this has anything to do with visibility.
It sounds like there may be a problem with multiple IFR flights arriving close to each other on visual approaches without cancelling IFR.
They therefore must still be separated from each other by the Class D tower controllers who have no standards available to them in these ridiculously small Class D control zones. (Not so at the regional Class Ds where there is more airspace available to the controllers)
The solution is for the Tower to insist that all IFR flights are processed via an instrument approach. This means that they are surveillance separated in Class C airspace by Melbourne Centre and then processed one at a time, with the VFR traffic, by the Tower controllers.
If it is VMC then an IFR flight can cancel IFR and be processed as a VFR flight without the delay incurred by an instrument approach.
This was the situation in the old GAAP system where CASA decreed that when an IFR flight contacted a GAAP Tower the pilot's IFR status was automatically changed to VFR. This changed when CASA made the Zones Class D.
Unfortunately the CASA/Airservices system has not caught up with the airspace status change, otherwise Moorabbin would be operated in the same way as Avalon, Albury, Launceston and so on.